Add the build time as a metric

Since ICFY requires a build to analyse the bundle anyways, it might be useful to keep track of the build time as a metric.

This might help up notice when regressions occur to our build perf. So we could catch things [like this](https://github.com/Automattic/wp-calypso/pull/42829) early on.

The issue names no files or tests. Start by tracing ICFY's Webpack build and existing bundle-size metric reporting, then determine where elapsed build time can be recorded; done means the metric is emitted alongside the current measurements and can expose build-performance regressions.

Written by the indexing model from the issue text.